POD don't have access to internet with minikube
What is the error or exception you are getting while requesting that API ?Is it a DNS resolution issue ?
If it is a DNS resolution issue.Then check if KubeDNS or CoreDNS is running on your kubernetes cluster.If specified above is running then try pinging the host from your pod.
Edit 1:
Looks like the socket connection is getting closed while reading from the server.Try passing auto_reconnect argument while calling rtm_connect function.
slack_client.rtm_connect(auto_reconnect = True)
I created an opensource project with all adjustments for a working Slack bot in Minikube using slackclient:https://github.com/staticdev/k8s-python-slackbot